I've heard many people refer to a month gain on either end of the season but you can always stretch anything further. It all becomes a matter of cost and economics. I've tried the water barrel thing but you really don't get much unless you cover the walls with water barrels. If you don't have a backup heat plan for frost sensative crops you can loose your crop in one night so I'd advise at least a propane heater.

Two film layers, inflated, save a bit more heat and you could even plan to use row cover material within the greenhouse.

Three barrels in a greenhouse that size is not adequate to make much of a difference. A supplemental heater would help, although a small one may not do what you need. You really need to do some calculations to determine how much heat you need to supply to get the effect you want, and thus determine the minimum size heater that will work.

We also have an unheated greenhouse in chilly zone 4, augmented with lots of 5 gallon buckets painted black and filled with water. This has extended our growing season enormously.

I start cold-hardy plants like spinach, broccoli and salad mixes in late Feb/early March. I will start my tender seedlings tomorrow. This is more than a month ahead of my old schedule for the cold-hardy stuff and exactly a month ahead for the tender plants (which I used to start indoors under lights).

On the other side of the season, we keep tomatoes growing until Nov, enjoy salads into December, and grow carrots year-round. Basically we turned our 6 month outdoor season into a 10 month season.

We don't want a heater for a bunch of reasons, but when it's colder than I want I can adjust by moving containers into a warm part of the GH, covering things, using heat mats, or as a last resort, bringing containers in the house. This all seems to work very well and I have no desire to make the GH warmer.

Over time you will figure out the schedule. My best advice is to keep detailed notes so you can learn from year to year.

For next winter I'll add more barrels, I have many just hanging around. I didn't realize 3 weren't enough! I have this fear of something going wrong and a fire starting...I really really don't want to put heat unless absolutely necessary.

Karen in MT, I'm glad to see it's working for you. I was hoping to get similar useage from the GH-I think I'll model yours and take it from there. We're in Z6.